Photographic processing

ABSTRACT

In photographic processing apparatus, by-products are produced due to the chemical reactions which occur during the processing of photographic materials. It is known to remove some of these by-products in accordance with the area of photographic material processed and a knowledge of the average level of production of the by-products. This leads to inaccuracies in maintaining a fixed level of the by-products in the processing solutions. Described herein is a method of controlling a subsystem which removes by-products from the processing solutions by using data relating to the exposure given to a photographic material in the printing stage of the processing apparatus to calculate the amount of by-products produced so that they can be exactly removed from the processing solutions.

This invention relates to improvements in or relating to photographicprocessing.

It is common practice in photofinishing laboratories to use adensitometer to measure the optical transmission and reflectiondensities of test strips of photographic materials which have beenexposed to a well defined given level. These test strips are used toprovide data with which the process, in both paper and film processors,is kept under control.

Many printers, particularly the more sophisticated ones which may beleft unattended while working, are equipped with multipixel filmscanners. These scanners are effectively high resolution densitometerswhich are capable of yielding density data which is later used by theexposure control algorithm of the printer to calculate the requiredexposure which must be given to the print being made from the negativebeing scanned.

It is known in the industry that a separate scanner may be attached tothe end of either the film processor or paper processor, especially forblack-and-white materials. This scanner is used to perform processcontrol based on the density of the processed material.

It has also been realized that the printer's scanner is effectively anon-board densitometer which can be used to effect process controlmeasurements from process control test strips. This saves the extraexpense of having a separate densitometer in the laboratory solely forthe purpose of measuring test strips. Some commercially availableprinters take advantage of this fact.

Currently the paper processor and printer form one unit in minilabs withthe film processor separate. More recently, processing apparatus areappearing in which the film processor, printer and paper processor areintegrated into one unit. This new type of apparatus are very close totrue "coin-slot" operation where a non-skilled customer could simplyplace his film in a receptacle, place his money in the slot and thenreceive his prints and processed film a short while later.

In the following discussion, all examples given will refer to colourphotographic systems unless otherwise stated.

Two broad types of chemical reaction take place in a photographicprocess, namely:

(1) those which are in some way dependent on the amount of image formedon the exposed material; and

(2) those which are independent of the amount of image formed on theexposed material.

Development is a good example of the first type of chemical reaction,and can be referred to as being "image-dependent". The amount ofdeveloper molecules used up in processing a piece of photographicmaterial is related to the amount of latent image formed on it for givendevelopment conditions. Another example of an "image-dependent" chemicalreaction is the bleaching process.

Fixing, on the other hand, is an example of an "image-independent"chemical reaction. All the silver in the photographic material isremoved in a fixer bath and this amount is essentially the sameregardless of the amount of exposure given to the material.

In addition, we may recognise two classes of chemical constituent of aseasoned process solution, namely:

a) those which are produced as a by-product of the reaction, such ashalide ions or unreacted molecules of oxidized developer in thedeveloper solution, and

b) those which are depleted as a result of the reaction, such as thethiosulphate ion in the fixer.

The replenishment of chemicals which are depleted in a reaction which is"image-independent" may be accomplished by a measure of the area of thephotographic material being processed. This is the case with fixerswhere all the silver is removed from the material and is complexed withthiosulphate ion. Replenishment of thiosulphate in the fixer is easilyachieved by knowing what area of film or paper has been processed andthe amount of silver per unit area of the material being processed. Thistechnique is well-known in the industry and has been used for a longtime.

Current industry practice for the replenishment of developers inprocessing apparatus is to use a signal derived solely from the area ofmaterial passing through the developer to control pumps metering theflow of developer replenisher from a holding tank into the developerbath. It is assumed that all material being developed has been exposedto the same average level. The replenisher system therefore adds D ml ofdeveloper replenisher per unit area of material passing through thedeveloper, where D is an amount recommended by manufacturers fromexperience. This system gives satisfactory performance in processingapparatus with large tank volumes but performs less well in small tanks.

A recent trend in the photographic industry is the production of smallminilabs which take up very little space. Some companies make smallcolour photocopiers which produce copies on photographic paper and desktop models are becoming an increasingly likely possibility. It isexpected that these machines will suffer from inaccurate replenishmentof the developer if the current system of replenishment is used.

EP-A-0 381 502 describes a method of controlling developer replenishmentin paper processing apparatus by deriving a signal from the exposuregiven to the paper by the printer, using that signal to calculate thequantity of dye which will be formed on the print after processing, andhence calculating the amount of developer used up. The developer is thenreplenished accordingly.

A further problem which has been encountered in the industry is thereplenishment or replacement of systems which remove unwanted componentsfrom either the processing solutions or from the effluent produced bythe processing apparatus.

One such system which is commonly used employs silver recoverycartridges to remove silver from the effluent of the fixing bath. Thesecartridges include "steel wool" and work on the principle that iron inthe "steel wool" is replaced by silver. However, it is often difficultto know when the cartridge needs to be replaced with a fresh one. Forthis reason two such cartridges are usually put in series and acomparison of the silver concentration in the connection between the twocartridges is made to see when the silver level begins to rise. At thispoint the operator will deduce that the upstream cartridge is nearingexhaustion and will replace it with the downstream cartridge, thedownstream cartridge being replaced with a fresh one at the same time.Although this method works, it requires a measurement to be made (oftenby unskilled operators), and it is envisaged that more complicatedremoval systems will be required for processing apparatus in the future,both for process control and for ensuring that effluent conforms withsewer discharge legislation.

Furthermore, it is likely that the measurements required to test theperformance of these more advanced removal systems may be difficult,costly and possibly impractical for an unskilled operator working in aclean environment like a shop or an office.

It is therefore an object of the present invention to provide animproved method for controlling and maintaining a subsystem ofphotographic processing apparatus which effects the removal of animage-dependent chemical component from a processing solution or fromthe effluent before it is discharged.

According to one aspect of the present invention, there is provided amethod of controlling the removal of chemical species which areimage-dependent by-products of chemical reactions during photographicprocessing in photographic processing apparatus, the apparatus includinga printing stage in which a film strip is copied on to photographicmaterial and a processing stage, the method including deriving a signalrelated to the measured exposure given to the photographic material inthe printing stage, characterized in that the derived signal is used tocontrol the removal of the by-products produced during processing of theexposed material.

In this specification, the term "film strip" relates to both negativefilm and reversal film for use in both black-and-white and coloursystems.

More specifically, the amount of image formed on the print can becalculated from the transmittance data measured by the printer in theprinting stage using the technique as described in EP-A-0 381 502. Theamount of image can then be used to calculate the amount of by-productsproduced due to image-dependent chemical reactions, and hence control asubsystem which effects the removal of such by-products.

In the case of colour materials which use dyes as the image-formingsubstances, the amounts of some by-products generated are more closelyrelated to the amount of silver which was developed rather than theamount of dye produced. For example, one halide ion is released forevery silver ion which is developed. This is true for all manufacturers'products. There will, however, be variations between differentmanufacturers' materials in the relation between the amount of developedsilver and the amount of dye produced after development. Relationshipsbetween exposure, developed silver and dye amounts, part of a largerbody of information usually referred to as sensitometric data, arereadily available from the manufacturers although typical values may beused with little loss in accuracy.

Information relating to the optical and chemical characteristics ofphotographic materials, such as, spectral sensitivities, dye spectralabsorption curves and relationships between optical density, developedsilver and exposure, will be termed sensitometric data. From thissensitometric data and the well-known chemical equations governingprocessing reactions, all of which may be stored in the control systemof the photographic processing apparatus, all important parametersconcerning the generation of image-dependent by-products may be easilycalculated from the measured exposure data using well-known techniquesfound in any textbook, for example, "The Theory of the PhotographicProcess", 4th Edition, published by Macmillan.

In accordance with the present invention, only by-products produced inrelation to the amount of image formed are to be controlled. By-productswhich are image-independent are usually controlled using the well knownprinciple of measuring the area of photographic material processed.

The method described herein uses a signal derived from the photographicprinter which exposes the photographic material such that it relatesdirectly to the amount of exposure given. This signal is thentransmitted down a link to the processor where it is converted and usedto control the replenishment and removal systems built into theprocessor. Additionally, the control of these systems will also requireother information such as development time and temperature of thesolutions. These parameters are normally readily available in mostcommercial processors.

For a better understanding of the present invention, the removal ofhalide ions from the developer bath will be considered by way ofexample.

Halide ions are produced in the developer bath as a by-product of thedevelopment reaction. The quantity of halide ions produced is related tothe exposure given to the photographic material being processed. Sincehalide ions act as a restrainer for the reaction, it is desired to keeptheir concentration at a predetermined level so as to maintain constantprocessing solution activity.

In this example, the processing apparatus incorporates a subsystem whichhas the ability of removing halide ions from the processing solution,the ions being removed by passing the processing solution over a coatedsubstrate to which the halide ions bind very strongly. For the purposesof this example, the reaction kinetics are sufficiently fast so that thehalide ions are bound to the substrate much faster than they areproduced in the developer. For a desired concentration of halide ions inthe developer of H moles per liter, and a piece of photographic materialwhich will produce h moles of halide ions (evenly distributed throughoutthe solution) when processed, the volume of liquid, v, can be calculatedfor which h moles of halide ions are present and where the totalsolution volume before development is V. Normally, photographicmaterials carry out a small amount of liquid with them as they pass fromone bath to another, and if it is assumed that the solution volumecarried out of the developer by the photographic material is c liters,the following equation is obtained:

    v=h(v-c)/(HV+h)

If volume, v, of liquid is removed from the developer and passed throughthe removal system for sufficient time to remove all the halide ionsbefore it is added back into the solution, the halide concentration inthe developer may be kept constant. The parameters H, V and c are knownconstants and h may be calculated from a knowledge of the exposure givento the photographic material, and hence v may be calculated. Thus a flowcontroller may be operated to dispense v liters of liquid into thehalide removal system. This example demonstrates how exposureinformation can be used to control the operation of the removal system.

It is noted that h is a function of the exposure given to the material,and may be determined from the sensitometric data relating to thephotographic material which is stored in the processing apparatus.Specifically, the relation between exposure and developed silver wouldbe used, since the number of halide ions released into the developersolution is identical to the number of silver ions developed to formmetallic silver.

If the capacity of the removal system is R moles of halide ions, it is asimple matter to predict when it will be exhausted. If T is the volumeof solution which can be treated:

    T=Rv/h

Thus the operator may be automatically alerted when action needs to betaken to change or replenish a removal system cartridge.

In all the discussion above, the exact form of the relation betweenhalide ions released and exposure is not the key issue, as it merelyserves to illustrate the principle that a calculation is possible.Neither is it necessary to use a "batch type" removal system asdescribed above. A more complicated, continuous flow system may be used,provided its characteristics are well-known and that accurateflow-measurement is possible.

Furthermore, the exact relation between measured exposure and the amountof any by-product generated during processing may be determinedexperimentally using techniques familiar to any one skilled in the artof printing and processing. Look-up tables of this empirical data maythen be used by the control system of the processing machine to controlthe removal subsystems built into it.

In the above discussion, it has been assumed that for every copy madefrom images on the film strip on to the photographic material in theprinter, a measure of the exposure would be made for the purpose ofcontrolling the removal systems in the processor in response to theby-products generated in processing each copy. This represents an idealsituation. For reasons of practicality, it may be preferable toaccumulate the measured exposure from a batch of copies and performactions to control the removal systems after each batch of copies hasbeen processed.

For example, some minilab printers expose a number of prints and thenprocess them batchwise. In the case of high speed printers, a whole rollof prints would be exposed and stored before being transferred to aprocessing machine.

For reasons explained in EP-A-0 381 502, it may prove to be mosteffective, especially when the printer and processor are physicallyseparated, for measured exposure data to be recorded on the back of eachprint in some coded form, for example, a bar code or punched holes, tobe read by the processing machine at the time of processing, and usedfor controlling chemical replenishment as described in EP-A-0 381 502,or, as in this case, chemical removal systems. The exposure data mayalso be stored on a separate medium, such as a magnetic disk, and thentransferred to the processor with the prints. It would then be read bythe processor while the prints are being processed.

Another variant on the present invention is to use a combination ofreplenishment by area and replenishment by calculation. In this case,the processor would normally replenish according to the area of paperprocessed using an average value per unit area for the replenishmentrate (subsequently referred as an "area-dependent" value). At the sametime it would continually calculate the correct amount of replenishmentbased on measured transmittance values of images to be copied and obtaina difference between the calculated and actual replenishment rates. Whenthe accumulated difference between the two replenishment rates exceeds athreshold level, a correction is made to the actual replenishment ratebased on the accumulated difference. For example, in the case of removalof halide ions, when the printer, based on calculation of the halideions released during processing, had accumulated a correction to thenormal removal rate greater than a threshold level, it would effect theappropriate correction at the next opportunity. This correction could,of course, be either a positive or negative amount.

Another important consideration is the spatial resolution of theexposure measurement made in the printer. Printers which use discretephotocells for determining exposure measure only the averagetransmittance of the film strip. In the case of a negative, a subjectcomprising a white spot against a black background would print as ablack spot on a white background. The black spot would have reached themaximum density the photographic material, in this case photographicpaper, could give. The amount of dye in the spot would therefore be lessthan that expected from a calculation based on the average transmittanceof the negative. Consequently, the calculated amount of by-productsgenerated in processing would be too great.

This can be overcome by the use of a higher resolution measurement ofthe transmittance of the negative. A scanning device, for example, acharge-coupled device having a 30 by 20 array would yield 600measurements of the transmittance of the negative. Areas of low densityon the negative which would give an area of D_(max) on the print couldbe recognized as such, by using the paper's reflection density versuslog(exposure) curve provided by the manufacturer. The dye amounts formedat each of the 600 areas could be added together to give an accuratecalculation of the total dye amount formed on the print.

The ultimate extension of this technique would be to apply it to ascanning printer where the negative is scanned at very high resolution.

The method according to the present invention is applicable to anyremoval system used in photographic processing apparatus whether it bebased on chemical binding, as above, or ionic replacement as inion-exchange columns and silver recovery cartridges or any other methodwhere an element of the system is either exhausted or needs replenishingwith reagent.

This method has the advantage that an indication can be given to anoperator when a removal system is nearly exhausted. This enablesmaintenance to be carried out at the right time and without the need forroutine measurements by the operator. Sometimes it is very difficult foran unskilled operator to make these measurements especially where theyare concerned with effluent discharge limits which may be very low.

Another advantage of this method is that automatic replenishment ofremoval systems may be achieved such that their removal efficiency ismaintained at a constant level.

For example, a liquid reagent which reacts strongly with the halide ionsmay have been chosen to cause the ions to precipitate out of thesolution as an alternative to using a solid substrate to which thehalide ions bind. In this case, the removal system may comprise aseparate reaction vessel in which known amounts of developer solutionare added to the liquid reagent. It is clear that the liquid reagentwould need replenishing from time to time in order to keep its activityhigh. This replenishment could be controlled by knowing the amount ofreagent used up in removing the halide ions. This amount is related tothe amount of halide ions to be removed which, in turn, may becalculated from the amount of exposure given to the photographicmaterial which released the halide ions.

In this above example, the liquid is reagent is the consumablecomponent.

Therefore, it can be seen that in addition to controlling the operationof removal systems for image-dependent chemical species, the presentinvention may also be used to control the replenishment of the removalsystem itself.

In the case of non-replenished removal systems, the present inventioncan be used to predict exhaustion of the removal system and provide asignal to alert an operator or an automatic system to take the necessarymaintenance actions. For example, in an automatic system, the signalcauses an actuator to switch over from a nearly-exhausted removal systemto a fully replenished system connected in parallel.

Furthermore, control of the concentration of components of the processproduced as by-products of chemical reactions which are image-relatedcan be provided without the need for chemical sensors being present inthe processing solution.

Moreover, for chemical species for which no convenient chemical sensorexists, the method of the present invention makes process andenvironmental control possible for the first time.

In the minilab environment, where each negative is measuredautomatically before it is printed, the exposure data may be easilyobtained with no extra hardware cost and with only a small softwareoverhead. The link between printer and processor is already there.

Naturally, other "image-dependent" by-products can also be removed usingthe method according to the present invention--in particular, oxidizeddeveloper molecules.

I claim:
 1. A method of controlling means for removing image-dependent by-products of chemical reactions produced during processing of silver halide photographic material in a photographic processing apparatus, the apparatus including an exposing section in which an image to be copied is exposed onto the photographic material and a processing section for processing the exposed photographic material, the method including the steps of:deriving a signal related to the exposure given to the photographic material during exposure in the exposing section; and using the derived signal to control said means for removing the by-products produced during processing of the photographic material.
 2. A method according to claim 1, wherein the derived signal is used to calculate the amount of by-products produced during processing of the exposed material.
 3. A method according to claim 1, wherein the by-products are ions.
 4. A method according to claim 3, wherein the ions are halide ions.
 5. A method according to claim 1, wherein the by-products are molecules.
 6. A method according to claim 5, wherein the molecules are oxidized developer molecules.
 7. A method according to claim 1, wherein said means for removing includes reagents used for effecting the removal of the chemical species which are image-dependent by-products of chemical reactions produced during processing of the silver halide photographic material in the photographic processing apparatus and the derived signal is used to control the rate of replenishment of the reagents in said means for removing.
 8. A method according to claim 7, wherein the derived signal is used to calculate the level of depletion of the reagents.
 9. A method according to claim 8, wherein the calculated level is used to provide a first signal to indicate near-exhaustion of the said means for removing.
 10. A method according to claim 9, wherein said means for removing comprises a first removal apparatus and a second removal apparatus and a first signal indicating near-exhaustion is used to switch between said first removal apparatus and said second removal apparatus.
 11. A method according to claim 1, wherein the signal is derived from measurements of the average transmittance of the image to be copied.
 12. A method according to claim 1, wherein the signal is derived from measurements of the average transmittance of a plurality of different small areas of the image to be copied.
 13. A method according to claim 11, wherein the signal is derived-from the sum of measurements of the average transmittance of a batch of images to be copied onto photographic material at the exposing section.
 14. A method according to claim 11, wherein the signal is further derived from data relating to the sensitometric characteristics of the photographic material.
 15. A method according to claim 1, wherein the derived signal is related to the amount of by-products to be removed by an empirical function.
 16. A method according to claim 1, wherein the derived signal is used to provide a correction to other control signals derived solely from a measurement of the area of the photographic material which has been processed.
 17. A method according to claim 1, wherein said means for removing comprises a flow controller for controlling flow of a processing solution from said processing section to said means for removing, the derived signal being utilized to control the flow from the flow controller.
 18. A method according to claim 17, wherein said means for removing further comprises a solid substrate over which processing solution is directed, the by-products binding to the substrate for removal from the processing solution.
 19. A method according to claim 18, wherein said means for removing operates in a batch mode.
 20. A method according to claim 18, wherein said means for removing operates in a continuous mode. 